Fiction, pure and simple. They pulled the temperatures out of their ass. When you assume 3C per doubling and then say oh look, it was 12 degrees hotter back here when CO2 was at 4800, you're engaging in circular reasoning, just as they are when they state the same about today. It is an assumption being used to prove an assumption.
They didn't pull it out of their ass. They observed anoxic oceans (which happens near 40 degrees celcius). They observed plantlife that migrated to the north. They observed no ice anywhere on the planet. They observed forests near the poles. Types of carbonates changed (temperature dependent). All of that indicates high temperatures.
Also, it is not a circular reasoning. The temperature rise is given by our understanding of physics, based on models that are pretty robust. Based on lab-measurements for the absorption spectra of CO2, H2O. Based on measurement of current radiation from the earth's surface. This is independent of the paleo-record.
But of course... the exact temperatures and CO2 concentrations at the time are an unknown. We can only guess at them, but those values don't look unreasonable to me. Those values of CO2 can be attained by extensive volcanic activity during a long, long period of time.
The thing that's scary to me is, that it ONLY takes a little over 10 degrees celcius of warming to get close to extinction levels. Fortunately for us, that takes a LOT of CO2 so it won't happen overnight, it'll take hundreds of years to reach that point.
You've bought into the idea that water vapor will advance the rate of warming, but why would this be so? Clouds increase our albedo. Latent heat in the water cycle accounts for over 20% of the energy loss.
Well, it's pretty obvious if you think about it. H2O molecules ("water vapor") in the air capture heat. When the air becomes warmer, it can contain more H2O. For example the poles are arid, because the air is so cold there is hardly any water in the air. If the poles become warmer, the air there can contain more H2O. The upper parts of the troposphere are also pretty cold, if they warm a little, the upper part can contain more H2O.
CO2 actually does increase heat with little in the way of additional cooling provided by conduction.
Convection distributes a temperature rise more efficiently over the atmosphere. It cannot prevent the capture of energy of the atmosphere. It cannot prevent warming of the atmosphere. Also, convection stops at 10 km height (or something, I don't remember the exact height of the temperature inversion) - at that point energy is lost by radiation from clouds and CO2. Convection cannot carry heat all the way into space...
If the increased temperature amplifies the water cycle, that results in just the opposite. More rain transporting heat out of the troposphere. More cloud cover blocking it from entering to start with.
Of course you are right that if there are more clouds, then heating could be mitigated. But there already many clouds on Earth, and they are usually concentrated in depressions while the rest of the skies are clear. Maybe clouds become bigger and contain more rain, but will the total cloud cover of the earth be significantly larger than it is now? Maybe if depressions grow larger, maybe they can cover a larger area with clouds, but it's a very small percentage of the total area of the earth.
Rain does not transport heat out of the troposphere... only radiation loss from H2O in the clouds and CO2, not the raindrops.
And it's not just the water cycles, it's also the H2O concentration in the air that's important. More H2O in the air means that heat can be trapped longer.
but the geological record that shows trailing CO2 trend lines throughout the various ice ages.
I'm not going into that again... as far as I'm concerned you cannot make such a bold conclusion from those data.
On top of that, ice ages are quite different events, they're controled by a tipping point after which a runaway cooling occurs. We're now in a situation were there is far less ice, ice does not control our climate anymore. We've to search for other explanations for warming and fortunately our understanding of how greenshouse gases work helps us a lot.
If CO2 is such an outstanding greenhouse gas, why do deserts still get so fucking cold at night?
Because clouds and H2O are not present. A lot of heat can escape at night. CO2 in itself is not enough of course.
Death Valley(which has a low DTR thanks to it's elevation) has a century old world record temperature of 134F and hasn't hit 130F since. With almost no humidity or cloud cover, and atmospheric conditions that block conduction, this place should be a test case for AGW, yes?
Maybe, if all local factors (like the geography) are taken into account as well, then who knows ... but I don't know much about that place, never been there. Maybe I'll read some more about it.
I'll read your link later on.